1. Technical Field
The present invention relates to an electric leakage sensing apparatus that is used to sense an electric leakage of a DC power supply in an electric automobile, for example.
2. Related Art
A high-voltage DC power supply is mounted on the electric automobile in order to drive a motor and an in-vehicle instrument. The DC power supply is electrically insulated from a grounded vehicle body. However, when an insulation failure or a short circuit occurs between the DC power supply and the vehicle body for any cause, a current flows through a route from the DC power supply to the ground to generate an electric leakage. Therefore, an electric leakage sensing apparatus that senses the electric leakage is provided in the DC power supply.
There is well known an electric leakage sensing apparatus in which a coupling capacitor is used. The electric leakage sensing apparatus includes: a coupling capacitor in which one end is connected to a DC power supply; a pulse generator that supplies a pulse to the other end of the coupling capacitor; a voltage detector that detects a voltage at the coupling capacitor charged by the pulse; and an electric leakage determination part that compares the voltage detected by the voltage detector to a threshold value, and makes a determination of existence or non-existence of the electric leakage based on the comparison result. Japanese Unexamined Patent Publication Nos. 2005-127821 and 2007-163291 disclose the electric leakage sensing apparatus having such a configuration.
In a technology disclosed in Japanese Unexamined Patent Publication No. 2007-159289, a power supply circuit boosts a charge voltage of a smoothing capacitor by use of energy accumulated in the smoothing capacitor in a power supply part of a motor drive device, and degradation of an insulating property of a motor is detected by applying the boosted voltage between a motor coil and a ground.
When the existence or non-existence of the electric leakage is sensed based on the voltage at the coupling capacitor, sensing accuracy depends on magnitude of the voltage applied to the coupling capacitor. That is, as illustrated in FIG. 4, in the case of a small voltage Va at the coupling capacitor, a variation range a of the capacitor voltage decreases when an electric leakage resistance between the DC power supply and the ground varies by a temperature and the like. That is, there is a limitation to the sensing accuracy because a dynamic range cannot be widened. On the other hand, as illustrated in FIG. 5, in the case of a large voltage Vb at the coupling capacitor, a variation range β of the capacitor voltage increases with respect to the variation in electric leakage resistance. That is, the sensing accuracy is improved because the dynamic range can be widened.
Accordingly, in order to enhance the sensing accuracy, it is necessary to provide a booster circuit, and to apply the voltage boosted by the booster circuit to the coupling capacitor. However, when the voltage at the coupling capacitor increases, the increased voltage exceeds a voltage (for example, 5 V) that can be read by a CPU, and the CPU cannot make the determination of the existence or non-existence of the electric leakage.